Pen type drug delivery devices have applications where regular injection by persons without formal medical training occurs. This is increasingly common among patients having diabetes or the like. Self-treatment enables such patients to conduct effective management of their disease. The injection pens usually comprise a housing in which the drive mechanism is located. Some kinds of drug delivery devices also comprise a compartment to accommodate a cartridge in which the medicament is received. With the drive mechanism, the bung in the cartridge is displaced for dispensing the medicament accommodated therein. The drive mechanism includes a piston rod that has a bearing at one end, wherein the bearing is arranged in such manner such that it faces the bung. With the piston rod, the bearing is displaced toward the bung and urges the bung toward a distal end of the drug delivery device, which is closest to the dispensing end (needle end) of the device. Medicament from the cartridge is dispensed thereby. The opposite side of the device is referred to as the proximal end.
In devices of the generic kind, the manufacture may bring unavoidable tolerances and functional clearances between the single components of the drug delivery device, in particular the drive mechanism. As a consequence, clearances such as a gap between the elements of the drive mechanism, such as between the bearing and the cartridge bung may occur even after the drug delivery device has been assembled so that the bung may not be in contact with the distal end of the bearing. It is, therefore, important to eliminate the gap between the cartridge bung and the distal end of the bearing and to bring the drive mechanism in a prestressed state prior to use. Otherwise, it would be possible that the dialed dose may not be dispensed from the device correctly. Initial clearances may already falsify the setting of the dose. To adjust the drug delivery device for use, priming actions are conducted to ensure that the drive mechanism is correctly adjusted, e.g. that the drive mechanism is in contact with the bung so that the correct amount of the medicament can expelled from the device. These actions often come along with a small amount of medicament being dispensed which gives a visual indication that the drug delivery device is ready to use.
It is known in the art to conduct adjustment of the drug delivery device by measurement of the bearing and the bung position before pressing, resp. assembly. The parts are then adjusted according to the measured value such that the bearing is brought into contact with the bung. However, the assembly machines for this method are expensive and the required time cycle is very long.